Apparatus to control the energy distribution of an electron beam



w. w. HUNT, JR 3,529,198 APPARATUS TO CONTROL THE ENERGY DISTRIBUTION OFAN ELECTRON BEAM Filed April 10, 1968 f Sept. 15, 1970 INVENTOR.W/AA/d/Y IMHO/V7 M. BY 4 United States Patent Oflice 3,529,198 PatentedSept. 15, 1970 3,529,198 APPARATUS TO CONTROL THE ENERGY DISTRIBUTION OFAN ELECTRON BEAM William W. Hunt, Jr., Chelmsford, Mass., assignor tothe United States of America as represented by the Secretary of the AirForce Filed Apr. 10, 1968, Ser. No. 720,263 Int. Cl. H01j 29/46 US. Cl.313-85 2 Claims ABSTRACT OF THE DISCLOSURE Apparatus to narrow theenergy distribution of an electron beam wherein an electron reflectorreceives at the input thereof an electron beam. The electrons of thebeam are forced to geometrically undergo successive collisions with thereflecting surface of the reflector. The output electron beam from thereflector has a substantially narrower energy distribution than theinput electron beam.

BACKGROUND OF THE INVENTION This invention relates to a method andapparatus for controlling an electron beam, and more particularly, to amethod and apparatus for using the energy (velocity) dependence of theelectron reflection coefficient to produce an electron beam having anarrower energy (velocity) distribution than the original source beam bygeometrically forcing the electrons to undergo successive collisionswith a reflecting surface.

Many devices such as electron microscopes, radar display tubes,television picture tubes, electron diffraction instruments, etc., arelimited by the energy spread of the electron beams used. In the priorart, there exists the technically diflicult problem of producinglow-energy electron beams with narrow energy (velocity) spreads and inthe past complex apparatus has been necessary to solve the problem.

SUMMARY OF THE INVENTION The reflection coeflicient R of a given surfacefor lowenergy electrons (say for E Z-S ev.) is known to depend on theenergy E with which the electrons hit the surface. Also, in this sameenergy range, the reflected electrons retain essentially all of thisincident energy. [See for instance, Niedermeyer & Holzl, Phys. Stat.Sol. 11, 651 (1965).] The reflection coefficient R(E) expresses theprobability that an electron striking the surface with energy between Eand E+dE will be reflected rather than absorbed.

If a beam of electrons having an energy distribution I(E) [such that1(E) dB is the number of electrons crossing a plane normal to the beamcurrent in the unit time with energy between E and E+dE and such that iscaused to strike a surface then some of the beam will be reflected andsome will be absorbed. The reflected current is given by and the energydistribution of the reflected electrons is now R(E)I(E). If thereflected beam is in some way forced to impinge again on the samesurface, then the current after this second reflection will be given byand the energy distribution of the electrons in this now twice-reflectedbeam is R (E)xI(E). Similarly, after n, such reflections the current isand the energy distribution is R(E)I(E).

If the reflection coeflicient undergoes maxima and minima as theincident energy (velocity) is increased from zero, then the electronenergy (velocity) distribution of a multiply-reflected (diffracted) beamwill have sharply defined peaks at energies (velocities) corres pondingto each maxima in R overlapped by the initial energy (velocity)distribution. If the positions of these maxima in R are known inadvance, then the original energy (velocity) distribution can be shiftedby means of accelerating or retarding voltages to give significantoverlap with only one of the maxima. In this case, the energy (velocity)distribution of the reflected electrons will be sharply peaked at theone energy (velocity) corresponding to this maximum, and the sharpnessof the peak will increase as n, the number of reflections, is increased.Even if R has no maxima the energy .(velocity) distribution of thereflected electrons will become sharper and sharper at that energy(velocity) corresponding to maximum overlap of R and I(E).

In accordance with the present invention an apparatus is provided sothat electrons having the desired initial energy distribution are causedto impinge on one end of the inner surface of a tube (which may have anydesired cross sectional configuration-i.e. it is not necessarily acylindrical tubeit may be two closely spaced plates and thus not even aclosed tube). After being reflected by the inside surface of the tubemany times, the electrons emerge from the other end of the tube with thenarrowed energy distribution described above. They can then beaccelerated, decelerated, or focussed as desired.

An object of the present invention is to provide an apparatus toproducing low-energy electron beams with narrow energy spreads.

Another object of the present invention is to provide an apparatus forusing the energy dependence of the electron reflections to produce anelectron beam having a narrower energy distribution than the originalsource beam.

Yet another object of the present invention is to provide an apparatusto narrow the energy distribution of an electron beam by geometricallyforcing the electrons of the beam to undergo successive collisions witha reflecting surface.

Yet another object of the present invention is to provide an apparatusfor narrowing the energy distribution of an electron beam by utilizingthe energy dependence of the reflection coeflicient of a surface forslow electrons in combination with multiple reflection from thereflective surface.

The various features of novelty which characterize this invention arepointed out with particularity in the claims annexed to and forming partof this specification. For a better understanding of the invention,however, its advantages and specific objects obtained with its use,reference should be had to the accompanying drawing and descriptivematter in which is illustrated and described a preferred embodiment ofthe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the single drawing of thepresent invention there is shown electron beam source 1 which may be anyconventional electron beam forming apparatus capable of providinglow-energy electrons, for example, where E535ev. In electron beam source1, there is shown voltage source 7 providing voltage by way oftransformer 8 for heating filament 9 which thereupon provides alowenergy electron beam emerging from 11. Variable voltage source 10 isutilized to control the intensity of electron beam 2. The output ofelectron beam source 1 is electron beam 2 which has a broad and thedesired initial energy distribution. Tube 3 having inner electronreflecting surface 4 is provided. Tube 3 may have any desiredcross-sectional configuration. In place of a cylindrical tube, there maybe utilized two closely spaced plates and thus not even a closed tube isnecessary. Tube 3 includes input end 5 and output end 6.

Electron beam source 1 is positioned so that the electrons of electronbeam 2 are directed into input end 5 of tube 3 so as to impinge at apredetermined angle upon inner surface 4 for reflection therefrom insuch manner as to geometrically force the electrons to undergosuccessive collisions with the reflecting surface without resort toelectrostatic or magnetic means. After being reflected by inner surface4 many times as indicated, the electrons emerge from output end 6 in theform of an electron beam with the desired narrower energy distributionthan input electron beam 2 as theoretically described hereinbefore.After emergence from output end 6, the electrons can then beaccelerated, decelerated, or focussed, as desired in accordance with anyof the conventional uses of an electron beam. It is to be noted thatalthough the structure is not indicated that the apparatus shown in thedrawing is attached in the conventional manner to provide the necessaryoperating conditions for an electron beam such as for example in anoperative vacuum and suitable conventional electrodes.

Thus an apparatus has been provided for using the energy dependence ofthe electron reflection coeflicient to produce an electron beam having anarrower energy distribution than the original source beam. This hasbeen accomplished by geometrically forcing the electrons to undergosuccessive collisions with reflecting surface 4.

In a constructive test of the apparatus of the present invention usingliterature values of R(E) for polycrystalline Pt [see Niedermeyer &Holzl, Phys. Stat. Sol. 11, 651 (1965)] combined with an initial energydistribution corresponding to normal component of a 3000 K. thermionicenergy distribution accelerated by 0.1 volt gave the reduced half-widthsand attenuations indicated after 0, 1, 3, 5 and 7 reflections (here W(0)=0.175 ev.) This is a severe test since neither the energydistribution nor this following particular set of reflection data hasmaximum of the type frequently reported. The halfwidth would continue todecrease if 41 were made larger. Reflection data is as follows:

The apparatus of the present invention in effect operates as anon-magnetic reflection type electron velocity filter and provides anovel use of the energy (velocity) dependence of the reflectioncoefficient of a surface for slow electrons in combination with the useof multiple reflections thereby solving the difficult problem ofproducing low-energy electron beams with narrow energy (velocity)spreads in much simpler manner.

Although I have shown and described a specific embodiment of myinvention, I am fully aware that many modifications thereof arepossible. My invention therefore is not to be restricted except insofaras is necessitated by the prior art and the spirit of the appendedclaims.

What is claimed is:

17 Apparatus to control and narrow the energy distribution of anelectron beam, reflective surfaces having an input end and an outputend, said reflective surfaces being at zero potential and free ofelectric and magnetic fields, an electron beam source providing alow-energy initial electron beam having a wide energy distribution, saidelectron beam source being positioned to direct the initial electronbeam into said input end to impinge at a predetermined angle upon one ofsaid reflective surfaces for reflection therefrom in such manner as togeometrically force the electrons to undergo successive collisions backand forth said reflective surfaces to emit from said output end aresultant beam with a substantially narrower energy distribution thansaid initial electron beam.

2. Apparatus to control the energy distribution of an electron beam asdescribed in claim 1 wherein said reflective surfaces is comprised of atube of preselected length.

References Cited UNITED STATES PATENTS 2,034,174 3/1936 Bruche 3l3l92,061,387 11/1936 Prinz 313- 2,210,034 8/1940 Keyston 31368 X 2,215,7799/1940 Clavier et al. 3l3-68 X RAYMOND F. HOSSFELD, Primary Examiner US.Cl. X.R. 313-83

